How to Do the impossible sit-up butt face smash prank

This is a great gym room prank. Do in the locker room as a freshman initiation or hoax idea. It takes a good actor to demonstrate the 'impossible sit-up' then a gullible person who believes after vigorous inhale one cannot sit-up. Then you need a dude to give the mark a bare ass to sit up into. Use this as inspiration for one of your April Fools Day pranks!